The present invention relates to polymeric films and, more particularly, to transparent polymeric films that provide a barrier to the transmission of ultra-violet light.
Polymeric films have numerous applications in the prior art. For example, many of these films may be formed such that they are transparent in appearance. Typically, these same films also possess an inherent moisture barrier and, as a result, make an ideal packaging material.
The prior art has previously addressed several problems associated with transparent polymeric films used for packaging . For instance, commonly-owned U.S. Pat. Nos. 4,419,411 and 4,590,125 disclose polypropylene films having low surface coefficients of friction and reduced tendencies to block (i.e., sticking of the film on the film roll), thereby providing improved packaging materials that are well-suited for automated packaging equipment. Commonly-owned U.S. patent application Ser. No. 07/662,661 discloses a film having improved thermal stripe adhesion which addresses the problem associated with heat sealing of polypropylene films.
As mentioned, many of the above-described films are used to package foods and, in particular, snack foods and the like. As is commonly known, snack foods have a limited shelf life. This limited shelf life is believed, in substantial part, to be caused by exposure of the food product to ultra-violet light, which is able to penetrate through the wrapping and contact the product.
Accordingly, by reducing the exposure of food products to ultra-violet light, the shelf life of such products would be increased. In this regard, it would be desirable to provide a transparent polymeric film that protects the food product from exposure to ultra-violet light.
Although the prior art is replete with films and film compositions, nowhere does the prior art disclose a transparent film that provides a barrier to the transmission of ultra-violet light. The prior art, instead, discloses various film compositions that provide a number of other desirable properties, including: (i) reduced film-to-film friction, (ii) reduced blocking, (iii) improved heat sealability, (iv) scratch resistance, (v) improved slipperiness, and (vi) enhanced opacity and brightness.
With respect to enhanced opacity and brightness, an inorganic compound such as titanium dioxide, which serves as a pigment, can be added to the film. This effect is disclosed in commonly-owned U.S. Pat. Nos. 4,560,614 and 4,741,950. The latter reference states that the pigment material, that is, the titanium dioxide, should be of such a size and shape that, when it is added to a polymeric material, it will not significantly contribute to void initiation of the polymer. To one skilled in the art, this would imply that titanium dioxide having an average particle size on the order of from about 0.050 .mu.m (microns) to 0.1 .mu.m be added to the film to produce the desired opacity and brightness effect. Additionally, titanium dioxide may be added to films to fill the voids formed by stretching (i.e., orientation of the film).
Moreover, titanium dioxide is known to have a marginal ultra-violet stabilizing effect when added to polymers in general. For example, in the past, titanium dioxide has been added to milk jugs. This, however, results in a noticeably white product because the titanium dioxide functions as a pigment. With respect to polymeric films, U.S. Pat. No. 4,780,402 states that when titanium dioxide particles having a size on the order of 0.5 .mu.m to 10 .mu.m are added to an opaque polyester film support, the titanium dioxide particle impose a yellowish coloration upon the film and, in addition, absorb ultra-violet light. The `402 titanium dioxide particles, however, are of a "pigment size", thereby rendering the film opaque in appearance and unsuitable for food packaging.